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Flame retardancy of polyisocyanurate–polyurethane foams: use of different charring agents
Polymer Degradation and Stability 78 (2002) 341–347 www.elsevier.com/locate/polydegstab
Flame retardancy of polyisocyanurate–polyurethane foams: use of different charring agents M. Modesti*, A. Lorenzetti Department of Chemical Process Engineering, Padova University, v. Marzolo 9, 35131 Padova, Italy Received 5 March 2002; received in revised form 21 May 2002; accepted 25 May 2002
Abstract The influence of different charring agents on physical-mechanical properties and fire behaviour of polyisocyanurate–polyurethane (PIR–PUR) foams has been investigated. In particular the use of varying amounts of ammonium polyphosphate, melamine cyanurate and expandable graphite has been analysed; all, when involved in fire, lead to the formation of a char layer on the polymer surface, but their ways of flame retardancy are different. The results obtained show that the higher the filler content the lower the compression strength; in particular the worst results have been obtained in the presence of melamine cyanurate. Moreover, the presence of ammonium polyphosphate or melamine cyanurate causes any significant worsening on thermal conductivity, while expandable graphite leads to a quite marked increase of the thermal conductivity. The fire behaviour has been studied by means of cone calorimeter apparatus and oxygen index test; it has been observed that the best results, i.e. the lowest rate of heat release and the highest oxygen index, are achieved with expandable graphite. Also the ammonium polyphosphate brings slight improvement in fire behaviour, whereas the effect of melamine cyanurate is negligible. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyisocyanurate–polyurethane foams; Ammonium polyphosphate; Melamine cyanurate; Expandable graphite; Char
1. Introduction The increasing awareness of public opinion toward the problem of fire safety of materials has led to the approval of new regulations [1] where toxicity and density of the smokes are very important factors that should be considered in evaluating fire safety. Therefore, there is today a need to find halogen-free flame retardants, as effective as the phospho-halogen ones. Such compounds, in fact, allow considerable improvement in the fire behaviour of the foams, but, on the other hand, cause the development of very dense and toxic smokes [2]. Therefore, in this work we have studied the influence of different halogen-free flame retardants on fire behaviour of PIR–PUR foams blown with n-pentane. Our attention has been put on charring agents: in particular we study the effect on fire behaviour of PIR–PUR foams due to the presence of various amounts of ammonium polyphosphate, melamine cyanurate and * Corresponding author. Fax: +39-049-8275555. E-mail address: [email protected] (M. Modesti).
expandable graphite. All those compounds lead to the formation of a superficial char layer that prevents further decomposition, but they act in three different ways: Ammonium polyphosphate (APP) leads to the formation of a char layer through the linking of phosphates to the ester group; the latter are readily eliminated forming conjugated double bonds, which finally cyclize to give char [3]; Melamine cyanurate (MC) acts through endothermic decomposition that leads to evolution of ammonia and formation of condensation polymers such as melam, melem, melom, which constitute the superficial char layer [4]. Two schemes of condensation have been proposed [5]: the first states that condensation leads to the fused-ring structure of cyameluric triamide which reacts as a trifunctional monomer to give the final condensate; the second states that the melamine unit is the trifunctional monomer which progressively condenses to give a product in which triazine rings are linked by –NH– bridges.
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00184-2
342
M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347
Expandable graphite (EG) leads to the formation of a char layer characterized by the presence of ‘‘worms’’, deriving from its expansion. According to some authors [6], the expansion of EG is due to a redox process between H2SO4, intercalated between graphite layers, and the graphite itself that originates the blowing gases according to the reaction: C þ 2H2 SO4 ! CO2 þ 2 H2 O þ 2 SO2 Therefore, the aim of this work is to analyse the effectiveness of such char layers, i.e. char formed by different interactions between filler and polymer. Moreover, the influence of the presence of flame retardants on the physical-mechanical properties of the foams has also been analysed.
2. Experimental 2.1. Raw materials The raw materials used were: Polymeric MDI (methane diphenyl diisocyanate): Tedimon 385 (Enichem): NCO%=30.5; average functionality=2.8. Polyester polyols: Glendion 9801 (Enichem): n OH=351 mgKOH/g, viscosity to 25 C=5600 mPa.s; Kitane (Enichem) n OH=435 mgKOH/ g, viscosity to 25 C=1100 mPas. Catalyst: pentamethyldiethylentriamine (PMDETA) (Abbot) and potassium octoate (K15) (Air Products). Surface-active agent: polysiloxane-polyether copolymer Tegostab B8469 (Goldschmidt-Italy). Blowing agent: n-pentane technical grade (Carlo Erba). Flame retardant: expandable graphite: Graf Guard 160–80 N, medium particle size 150 mm (Ucar Graph. Tech); melamine cyanurate: Budit 314, N=48%, particle size=5–10 mm (Budenheim R. A. Oetker); ammonium polyphosphate: FR Cross 484, P2O5=72%; N=14%, medium particle size =10 mm (Budenheim R. A. Oetker).
The foams were prepared by hand mixing technique, that is the isocyanate was added to the formulated polyol (i.e. the mixture of polyols, catalysts, surfactant, filler, blowing agent); then the mixture mixed for 15 s and poured into an aluminium cup. During the expansion several kinetic parameters (cream time, gel time and tack free time) were recorded. After the preparation the foams were put in a oven at 70 C for 24 h, in order to complete the polymerisation reaction, before carrying out physical-mechanical and fire behaviour characterisation. 2.3. Test methods For all the foams produced, we analysed both physical-mechanical properties and fire behaviour. The physical and mechanical properties were measured using standard test methods. In particular, the apparent density was measured according to ISO 845, the compression strength according to ISO 844 and the thermal conductivity according to ISO 8301. Fire behaviour was investigated by means of DIN 4102-B2 and oxygen index test according to ISO 3216. Moreover, the fire behaviour was analysed by using a cone calorimeter. This apparatus allows measurement of a wide variety of parameters, rate of heat release (RHR), effective heat of combustion (EHC), mass loss, smoke extinction area (SEA), development of CO and CO2. Among them, we take into consideration the RHR and CO/CO2 weight ratio, which are the more important parameters in order to understand the fire performance of a material: in fact, RHR peak value is believed by many fire scientists as the responsible for the ‘‘flashover’’ phenomena in a real fire situation [7] while CO/CO2 weight ratio, being an index of combustion completeness, can be considered as an index of smoke toxicity. At the moment, standard regulation exists only for RHR (ISO 5660), while for the other parameters the measurements are not standardized. In order to overcome problem of poor repeatability of data, five specimens for each sample were submitted to each kind of test. Therefore in the graphs representing experimental results the error bars are also reported.
3. Results and discussion 3.1. Physical and mechanical properties
2.2. Foam preparation The foams formulations are reported in Table 1. We have prepared PIR–PUR foams, blown with n-pentane, characterised by a constant NCO index (250) and filled with several amounts (0, 15, 25 wt.%) of APP, MC or EG. The amount of pentane was calculated in order to obtain foams with a constant density (35 4 kg/m3).
The data for foam characterisation are reported in Table 1. The results show that, for foams filled with MC or EG, the higher the filler content the lower the compression strength, as it is expected. This could be due, in case of MC, to the fact that the melamine cyanurate leads to an increase on polymer friability, as already observed in previous work [8], and therefore brings a
343
M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347 Table 1 Foams formulations, kinetic parameters and physical-mechanical properties Formulation
Ref.
AP15
AP25
M15
M25
EG15
EG25
Glendion 9801 Kitane PMDETA K15 Tegostab B 8469 H2O n-Pentane FR Cross 484 Budit 314 GRAF Guard 160–80N Tedimon 385
70 30 0.2 1.8 2.5 1.5 10 0 0 0 290
70 30 0.2 2.2 2.5 1.5 28 74.9 0 0 290
70 30 0.3 2.4 2.5 1.5 35 143.9 0 0 290
70 30 0.2 2.2 2.5 1.5 28 0 74.9 0 290
70 30 0.3 2.4 2.5 1.5 35 0 143.9 0 290
70 30 0.2 2.5 2.5 1.5 14.5 0 0 72.5 290
70 30 0.2 2.2 2.5 1.5 24 0 0 140.1 290
Cream time (s) Gel time (s) Tack free time (s)
38 75 94
32 120 155
34 105 125
35 110 130
23 88 102
24 47 54
30 75 100
Density (kg/m3) Thermal conductivity (mW/mK) Parallel comp. strength (kPa) Perpend. comp. strength (kPa)
35 26.2 210 90
35 26.4 177 131
35 26.6 184 107
32 27.0 146 68
32 26.9 126 78
39 28.6 186 66
38 30.6 164 48
Fig. 1. Particle of expandable graphite (marked by black circle) between cell walls.
Fig. 2. Particle of ammonium polyphosphate (marked by black circle) in cell struts.
worsening of compression strength. In the case of EG, although the effect is less significant than in case of MC, the worsening could be due to the fact that the filler does not locate in the struts but between the cell walls (Fig. 1) because of its great particle dimensions (150 mm). This, causing an inhomogeneous cellular structure, could be responsible for the lower values of compression strength. The thermal conductivity results show that the presence of EG, causing an increase on the mean cell size [9], leads to an increase of thermal conductivity. Moreover, the presence of either APP or MC does not significantly affect the thermal conductivity: in fact, as both APP and MC are characterized by mean particle size ( ffi 10 mm) smaller than the mean cell size, these fillers locate on the struts (Figs. 2 and 3) thus avoiding increase of the mean cell size.
Fig. 3. Particle of melamine cyanurate (marked by black circle) in cell struts.
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M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347
3.2. Fire behaviour
Table 2 DIN 4102-B2 test results Sample
Ref. AP15 AP25 M15 M25 EG15 EG25
Ignited on Corner
Surface
Not B2 Not B2 Not B2 Not B2 Not B2 B2 B2
Not B2 Not B2 Not B2 Not B2 Not B2 B2 B2
The fire reaction of filled PIR–PUR foams has been analysed by use of DIN 4102-B2 and oxygen index tests. The results of the DIN 4102-B2 test are reported in Table 2. Only the foams filled with EG, containing at least 15 wt.% of filler, can be classified as B2 materials. This is a very successful result as pentane blown foams can very seldom be rated as B2 materials; in fact, the foams filled with APP and MC, even if containing very high flame retardant amount (25 wt.%), can not be rated as B2.
Fig. 4. Oxygen index results.
Fig. 5. Maximum RHR values as a function of the filler amount.
M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347
The oxygen index (OI) test (Fig. 4) showed that the OI increases with increasing filler content. In particular, while the presence of MC does not significantly change the OI, the presence of APP or EG leads to an increase of about 25 and 35% respectively, using 25 wt.% of filler. The most important result from the RHR (Figs. 5 and 6) is the considerable decrease that is achieved in presence of 25 wt.% of EG: the maximum value of RHR lowers by about 60% and the mean value by about 80%; the results are also satisfactory in the presence of 15 wt.% of EG. Moreover, it has been observed that
345
both APP and MC are more effective at lower amount as the maximum RHR value is lower with 15 wt.% than with 25 wt.% of filler. It seems therefore that the flame retardancy of the foams does not increase continuously with the filler content but rather shows an optimum, as already observed by Piechota [10] for APP. No significant influence of either APP or MC on the mean RHR value is observed. The CO/CO2 weight ratio results are reported in Fig. 7. In the presence of EG the values of the ratio are fairly high, while in the presence of APP or MC the ratio becomes lower.
Fig. 6. Mean RHR values as a function of the filler amount.
Fig. 7. CO/CO2 average values as a function of the filler amount.
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M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347
Fig. 8. Polymer below the char layer formed by EG.
Fig. 10. Polymer below the char layer formed by MC.
are the lowest. Otherwise, the fire performances of APP or MC filled foams are less satisfactory, particularly for MC, as the char layers formed are less effective in preventing further decomposition of the polymer. In presence of MC, the only positive effect is the lowering of CO/CO2 weight ratio, that is probably due to a decrease in flame temperature due to the endothermic reactions accompanying the decomposition of MC, which favours the left-hand side of the equilibrium reaction: CO2 þ C * ) 2 CO
H ¼ þ172 kJ=mol
i.e. favouring the CO2 development.
Fig. 9. Polymer below the char layer formed by APP.
Besides fire behaviour characterisation, in order to further test the effectiveness of the char layer formed by the different charring agent, we analysed foams after the oxygen index test by means of SEM. In particular a section at 1.5 mm below the superficial char layer has been taken into account. As can be seen in Fig. 8, the char formed by expansion of EG is very effective in preventing decomposition of the material: beside the presence of worm like structure deriving from expansion of EG (that begins at 160 C), the polymer below the char layer is almost not decomposed. Otherwise, below the char formed by APP or MC the polymer is softened: the ‘‘drops’’ are clear evidence of polymer degradation (Figs. 9 and 10). This means that in the presence of EG the temperature reached underneath the char is almost lower than in presence of APP or MC, i.e. the char formed by EG is much more insulating and/or compact and therefore limits the heat and/or oxygen transfer to the polymer; therefore the effectiveness of the EG char layer is greater than that of APP or MC. As the EG allows a good fire protection of the polymer, it is clear that the results obtained for EG filled foams are better, that is the material can be classified as B2, the OI is the highest, the RHR, both maximum and mean value,
4. Conclusions The results obtained have clearly shown that the effectiveness of char layers formed by different charring agents is not the same. The best results, that is the best fire performances have been obtained in the presence of 25 wt.% of EG, although the fire behaviour is also satisfactory in the presence of 15 wt.% of EG. The only undesired effect observed is a slight increase on the incomplete combustion in the presence of a high amount (25 wt.%) of EG. The fire performances of APP or MC filled foams are, in general, worst, except for the CO/CO2 weight ratio. The physical-mechanical characterisation showed that the EG affects the physical-mechanical properties, and particularly the thermal conductivity, much more than APP or MC. Otherwise, the effect is more marked at higher filler content (25 wt.%). However the foams produced show suitable physical-mechanical properties.
References [1] Final Draft prEN 13501-1: Fire classification of construction products and building elements. Part 1: classification using test data from reaction to fire tests.
M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347 [2] Camino G, Luda MP, Costa L. In: Casal J. editor Proceedings of Chemical Industry and Environment. Universitat Politecnica de Catalunja; 1993. p. 221–7. [3] Kishore K, Mohandas K. Combustion and Flame 1981;43:145–53. [4] Taylor AP, Sale FR. Makromol Chem Macromol Symp 1993; 74:85–93. [5] Costa L, Camino G. J Thermal Anal 1988;34:423–4. [6] Camino G, Duquesne S, Delobel R., Eling B, Lindsay C, Roels T. In: Nelson GL, Wilkie CA, editors. Symposium Series N 797/
[7] [8] [9] [10]
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Fires and Polymers. Materials and Solutions for Hazard Prevention. Washington DC: ACS Pub.; 2001. p. 90–109 [Chapter 8]. Vanspeybroeck R, Van Hess P, Vandevelde P. Cell Polym 1992; 11(2):96. Modesti M, Lorenzetti A, Simioni F, Checchin M. Polym Degrad Stab 2001;74(3):475–9. Modesti M, Lorenzetti A, Simioni F, Camino G. Polym Degrad Stab (in press). Piechota, H. J Cell Plastics 1965;53:187–99.
Flame retardancy of polyisocyanurate–polyurethane foams: use of different charring agents M. Modesti*, A. Lorenzetti Department of Chemical Process Engineering, Padova University, v. Marzolo 9, 35131 Padova, Italy Received 5 March 2002; received in revised form 21 May 2002; accepted 25 May 2002
Abstract The influence of different charring agents on physical-mechanical properties and fire behaviour of polyisocyanurate–polyurethane (PIR–PUR) foams has been investigated. In particular the use of varying amounts of ammonium polyphosphate, melamine cyanurate and expandable graphite has been analysed; all, when involved in fire, lead to the formation of a char layer on the polymer surface, but their ways of flame retardancy are different. The results obtained show that the higher the filler content the lower the compression strength; in particular the worst results have been obtained in the presence of melamine cyanurate. Moreover, the presence of ammonium polyphosphate or melamine cyanurate causes any significant worsening on thermal conductivity, while expandable graphite leads to a quite marked increase of the thermal conductivity. The fire behaviour has been studied by means of cone calorimeter apparatus and oxygen index test; it has been observed that the best results, i.e. the lowest rate of heat release and the highest oxygen index, are achieved with expandable graphite. Also the ammonium polyphosphate brings slight improvement in fire behaviour, whereas the effect of melamine cyanurate is negligible. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyisocyanurate–polyurethane foams; Ammonium polyphosphate; Melamine cyanurate; Expandable graphite; Char
1. Introduction The increasing awareness of public opinion toward the problem of fire safety of materials has led to the approval of new regulations [1] where toxicity and density of the smokes are very important factors that should be considered in evaluating fire safety. Therefore, there is today a need to find halogen-free flame retardants, as effective as the phospho-halogen ones. Such compounds, in fact, allow considerable improvement in the fire behaviour of the foams, but, on the other hand, cause the development of very dense and toxic smokes [2]. Therefore, in this work we have studied the influence of different halogen-free flame retardants on fire behaviour of PIR–PUR foams blown with n-pentane. Our attention has been put on charring agents: in particular we study the effect on fire behaviour of PIR–PUR foams due to the presence of various amounts of ammonium polyphosphate, melamine cyanurate and * Corresponding author. Fax: +39-049-8275555. E-mail address: [email protected] (M. Modesti).
expandable graphite. All those compounds lead to the formation of a superficial char layer that prevents further decomposition, but they act in three different ways: Ammonium polyphosphate (APP) leads to the formation of a char layer through the linking of phosphates to the ester group; the latter are readily eliminated forming conjugated double bonds, which finally cyclize to give char [3]; Melamine cyanurate (MC) acts through endothermic decomposition that leads to evolution of ammonia and formation of condensation polymers such as melam, melem, melom, which constitute the superficial char layer [4]. Two schemes of condensation have been proposed [5]: the first states that condensation leads to the fused-ring structure of cyameluric triamide which reacts as a trifunctional monomer to give the final condensate; the second states that the melamine unit is the trifunctional monomer which progressively condenses to give a product in which triazine rings are linked by –NH– bridges.
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00184-2
342
M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347
Expandable graphite (EG) leads to the formation of a char layer characterized by the presence of ‘‘worms’’, deriving from its expansion. According to some authors [6], the expansion of EG is due to a redox process between H2SO4, intercalated between graphite layers, and the graphite itself that originates the blowing gases according to the reaction: C þ 2H2 SO4 ! CO2 þ 2 H2 O þ 2 SO2 Therefore, the aim of this work is to analyse the effectiveness of such char layers, i.e. char formed by different interactions between filler and polymer. Moreover, the influence of the presence of flame retardants on the physical-mechanical properties of the foams has also been analysed.
2. Experimental 2.1. Raw materials The raw materials used were: Polymeric MDI (methane diphenyl diisocyanate): Tedimon 385 (Enichem): NCO%=30.5; average functionality=2.8. Polyester polyols: Glendion 9801 (Enichem): n OH=351 mgKOH/g, viscosity to 25 C=5600 mPa.s; Kitane (Enichem) n OH=435 mgKOH/ g, viscosity to 25 C=1100 mPas. Catalyst: pentamethyldiethylentriamine (PMDETA) (Abbot) and potassium octoate (K15) (Air Products). Surface-active agent: polysiloxane-polyether copolymer Tegostab B8469 (Goldschmidt-Italy). Blowing agent: n-pentane technical grade (Carlo Erba). Flame retardant: expandable graphite: Graf Guard 160–80 N, medium particle size 150 mm (Ucar Graph. Tech); melamine cyanurate: Budit 314, N=48%, particle size=5–10 mm (Budenheim R. A. Oetker); ammonium polyphosphate: FR Cross 484, P2O5=72%; N=14%, medium particle size =10 mm (Budenheim R. A. Oetker).
The foams were prepared by hand mixing technique, that is the isocyanate was added to the formulated polyol (i.e. the mixture of polyols, catalysts, surfactant, filler, blowing agent); then the mixture mixed for 15 s and poured into an aluminium cup. During the expansion several kinetic parameters (cream time, gel time and tack free time) were recorded. After the preparation the foams were put in a oven at 70 C for 24 h, in order to complete the polymerisation reaction, before carrying out physical-mechanical and fire behaviour characterisation. 2.3. Test methods For all the foams produced, we analysed both physical-mechanical properties and fire behaviour. The physical and mechanical properties were measured using standard test methods. In particular, the apparent density was measured according to ISO 845, the compression strength according to ISO 844 and the thermal conductivity according to ISO 8301. Fire behaviour was investigated by means of DIN 4102-B2 and oxygen index test according to ISO 3216. Moreover, the fire behaviour was analysed by using a cone calorimeter. This apparatus allows measurement of a wide variety of parameters, rate of heat release (RHR), effective heat of combustion (EHC), mass loss, smoke extinction area (SEA), development of CO and CO2. Among them, we take into consideration the RHR and CO/CO2 weight ratio, which are the more important parameters in order to understand the fire performance of a material: in fact, RHR peak value is believed by many fire scientists as the responsible for the ‘‘flashover’’ phenomena in a real fire situation [7] while CO/CO2 weight ratio, being an index of combustion completeness, can be considered as an index of smoke toxicity. At the moment, standard regulation exists only for RHR (ISO 5660), while for the other parameters the measurements are not standardized. In order to overcome problem of poor repeatability of data, five specimens for each sample were submitted to each kind of test. Therefore in the graphs representing experimental results the error bars are also reported.
3. Results and discussion 3.1. Physical and mechanical properties
2.2. Foam preparation The foams formulations are reported in Table 1. We have prepared PIR–PUR foams, blown with n-pentane, characterised by a constant NCO index (250) and filled with several amounts (0, 15, 25 wt.%) of APP, MC or EG. The amount of pentane was calculated in order to obtain foams with a constant density (35 4 kg/m3).
The data for foam characterisation are reported in Table 1. The results show that, for foams filled with MC or EG, the higher the filler content the lower the compression strength, as it is expected. This could be due, in case of MC, to the fact that the melamine cyanurate leads to an increase on polymer friability, as already observed in previous work [8], and therefore brings a
343
M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347 Table 1 Foams formulations, kinetic parameters and physical-mechanical properties Formulation
Ref.
AP15
AP25
M15
M25
EG15
EG25
Glendion 9801 Kitane PMDETA K15 Tegostab B 8469 H2O n-Pentane FR Cross 484 Budit 314 GRAF Guard 160–80N Tedimon 385
70 30 0.2 1.8 2.5 1.5 10 0 0 0 290
70 30 0.2 2.2 2.5 1.5 28 74.9 0 0 290
70 30 0.3 2.4 2.5 1.5 35 143.9 0 0 290
70 30 0.2 2.2 2.5 1.5 28 0 74.9 0 290
70 30 0.3 2.4 2.5 1.5 35 0 143.9 0 290
70 30 0.2 2.5 2.5 1.5 14.5 0 0 72.5 290
70 30 0.2 2.2 2.5 1.5 24 0 0 140.1 290
Cream time (s) Gel time (s) Tack free time (s)
38 75 94
32 120 155
34 105 125
35 110 130
23 88 102
24 47 54
30 75 100
Density (kg/m3) Thermal conductivity (mW/mK) Parallel comp. strength (kPa) Perpend. comp. strength (kPa)
35 26.2 210 90
35 26.4 177 131
35 26.6 184 107
32 27.0 146 68
32 26.9 126 78
39 28.6 186 66
38 30.6 164 48
Fig. 1. Particle of expandable graphite (marked by black circle) between cell walls.
Fig. 2. Particle of ammonium polyphosphate (marked by black circle) in cell struts.
worsening of compression strength. In the case of EG, although the effect is less significant than in case of MC, the worsening could be due to the fact that the filler does not locate in the struts but between the cell walls (Fig. 1) because of its great particle dimensions (150 mm). This, causing an inhomogeneous cellular structure, could be responsible for the lower values of compression strength. The thermal conductivity results show that the presence of EG, causing an increase on the mean cell size [9], leads to an increase of thermal conductivity. Moreover, the presence of either APP or MC does not significantly affect the thermal conductivity: in fact, as both APP and MC are characterized by mean particle size ( ffi 10 mm) smaller than the mean cell size, these fillers locate on the struts (Figs. 2 and 3) thus avoiding increase of the mean cell size.
Fig. 3. Particle of melamine cyanurate (marked by black circle) in cell struts.
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M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347
3.2. Fire behaviour
Table 2 DIN 4102-B2 test results Sample
Ref. AP15 AP25 M15 M25 EG15 EG25
Ignited on Corner
Surface
Not B2 Not B2 Not B2 Not B2 Not B2 B2 B2
Not B2 Not B2 Not B2 Not B2 Not B2 B2 B2
The fire reaction of filled PIR–PUR foams has been analysed by use of DIN 4102-B2 and oxygen index tests. The results of the DIN 4102-B2 test are reported in Table 2. Only the foams filled with EG, containing at least 15 wt.% of filler, can be classified as B2 materials. This is a very successful result as pentane blown foams can very seldom be rated as B2 materials; in fact, the foams filled with APP and MC, even if containing very high flame retardant amount (25 wt.%), can not be rated as B2.
Fig. 4. Oxygen index results.
Fig. 5. Maximum RHR values as a function of the filler amount.
M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347
The oxygen index (OI) test (Fig. 4) showed that the OI increases with increasing filler content. In particular, while the presence of MC does not significantly change the OI, the presence of APP or EG leads to an increase of about 25 and 35% respectively, using 25 wt.% of filler. The most important result from the RHR (Figs. 5 and 6) is the considerable decrease that is achieved in presence of 25 wt.% of EG: the maximum value of RHR lowers by about 60% and the mean value by about 80%; the results are also satisfactory in the presence of 15 wt.% of EG. Moreover, it has been observed that
345
both APP and MC are more effective at lower amount as the maximum RHR value is lower with 15 wt.% than with 25 wt.% of filler. It seems therefore that the flame retardancy of the foams does not increase continuously with the filler content but rather shows an optimum, as already observed by Piechota [10] for APP. No significant influence of either APP or MC on the mean RHR value is observed. The CO/CO2 weight ratio results are reported in Fig. 7. In the presence of EG the values of the ratio are fairly high, while in the presence of APP or MC the ratio becomes lower.
Fig. 6. Mean RHR values as a function of the filler amount.
Fig. 7. CO/CO2 average values as a function of the filler amount.
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M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347
Fig. 8. Polymer below the char layer formed by EG.
Fig. 10. Polymer below the char layer formed by MC.
are the lowest. Otherwise, the fire performances of APP or MC filled foams are less satisfactory, particularly for MC, as the char layers formed are less effective in preventing further decomposition of the polymer. In presence of MC, the only positive effect is the lowering of CO/CO2 weight ratio, that is probably due to a decrease in flame temperature due to the endothermic reactions accompanying the decomposition of MC, which favours the left-hand side of the equilibrium reaction: CO2 þ C * ) 2 CO
H ¼ þ172 kJ=mol
i.e. favouring the CO2 development.
Fig. 9. Polymer below the char layer formed by APP.
Besides fire behaviour characterisation, in order to further test the effectiveness of the char layer formed by the different charring agent, we analysed foams after the oxygen index test by means of SEM. In particular a section at 1.5 mm below the superficial char layer has been taken into account. As can be seen in Fig. 8, the char formed by expansion of EG is very effective in preventing decomposition of the material: beside the presence of worm like structure deriving from expansion of EG (that begins at 160 C), the polymer below the char layer is almost not decomposed. Otherwise, below the char formed by APP or MC the polymer is softened: the ‘‘drops’’ are clear evidence of polymer degradation (Figs. 9 and 10). This means that in the presence of EG the temperature reached underneath the char is almost lower than in presence of APP or MC, i.e. the char formed by EG is much more insulating and/or compact and therefore limits the heat and/or oxygen transfer to the polymer; therefore the effectiveness of the EG char layer is greater than that of APP or MC. As the EG allows a good fire protection of the polymer, it is clear that the results obtained for EG filled foams are better, that is the material can be classified as B2, the OI is the highest, the RHR, both maximum and mean value,
4. Conclusions The results obtained have clearly shown that the effectiveness of char layers formed by different charring agents is not the same. The best results, that is the best fire performances have been obtained in the presence of 25 wt.% of EG, although the fire behaviour is also satisfactory in the presence of 15 wt.% of EG. The only undesired effect observed is a slight increase on the incomplete combustion in the presence of a high amount (25 wt.%) of EG. The fire performances of APP or MC filled foams are, in general, worst, except for the CO/CO2 weight ratio. The physical-mechanical characterisation showed that the EG affects the physical-mechanical properties, and particularly the thermal conductivity, much more than APP or MC. Otherwise, the effect is more marked at higher filler content (25 wt.%). However the foams produced show suitable physical-mechanical properties.
References [1] Final Draft prEN 13501-1: Fire classification of construction products and building elements. Part 1: classification using test data from reaction to fire tests.
M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347 [2] Camino G, Luda MP, Costa L. In: Casal J. editor Proceedings of Chemical Industry and Environment. Universitat Politecnica de Catalunja; 1993. p. 221–7. [3] Kishore K, Mohandas K. Combustion and Flame 1981;43:145–53. [4] Taylor AP, Sale FR. Makromol Chem Macromol Symp 1993; 74:85–93. [5] Costa L, Camino G. J Thermal Anal 1988;34:423–4. [6] Camino G, Duquesne S, Delobel R., Eling B, Lindsay C, Roels T. In: Nelson GL, Wilkie CA, editors. Symposium Series N 797/
[7] [8] [9] [10]
347
Fires and Polymers. Materials and Solutions for Hazard Prevention. Washington DC: ACS Pub.; 2001. p. 90–109 [Chapter 8]. Vanspeybroeck R, Van Hess P, Vandevelde P. Cell Polym 1992; 11(2):96. Modesti M, Lorenzetti A, Simioni F, Checchin M. Polym Degrad Stab 2001;74(3):475–9. Modesti M, Lorenzetti A, Simioni F, Camino G. Polym Degrad Stab (in press). Piechota, H. J Cell Plastics 1965;53:187–99.